Interleukin-1 (IL-a) is a polypeptide produced by human cells that has a wide range of biological effects and is involved in responses to inflammatory, immunologic and infectious challenges. The genes for two different forms of IL-1, IL-1 alpha and IL-1 beta, have been cloned allowing in vitro production of large quantities of these cytokines. IL-1 alpha has been shown to have immunoenhancing, bone marrow restorative and direct anti-tumor effects. Because of these effects, it was postulated that IL-1 alpha could have significant clinical utility in the treatment of malignant disease and a phase I study of its toxicity and hematologic and immunologic effects in humans was undertaken. My role has been to assess pre- and post-treatment bone marrow aspirates and peripheral blood smears in patients with metastatic malignant disease given increasing doses of IL-1 alpha.